Heretofore, a titanium oxide has been used as a catalyst carrier for, for example, reducing a nitrogen oxide and oxidizing an organic compound, wherein the carrier holds a catalyst component (e.g. a material selected from a metal such as V, W, Al, As, Ni, Zr, Mo, Ru, Mg, Ca and Pt, its oxide and its complex oxide.) The titanium oxide is also used as a photocatalyst or a carrier for photocatalyst. The titanium oxide is normally used in the form of a powder, a particle or pellet.
With a recent expansion in use of the titanium oxide, some characteristics have been required, but they can not be accomplished by the above conventional forms of titanium oxide. In order to cope with the requirements, various titania fibers have been developed.
However, these titania fibers have problems that the length of each fiber is about several millimeters (in other words, the fibers are so-called short fibers) and the mechanical strength of the fibers is not sufficient.
The following methods (1) to (7) have hitherto been known for producing titania fibers:
(1) a method comprising the steps of subjecting a potassium titania fiber as a starting material to a depotassiumation treatment in an acid solution and calcining the resultant to produce a titania fiber (see JP-A-53-41518, JP-A-53-52737, JP-A-55-113625, JP-B-59-41928, JP-A-1-246139 and JP-A-2-164722); PA1 (2) a method comprising the steps of spinning polytitanoxane to obtain a precursor fiber and calcining the precursor fiber to produce a titania fiber (JP-A-49-124336); PA1 (3) a method comprising the steps of concentrating an water-miscible titania sol obtained by adding a titanium alkoxide in a concentrated hydrochloric acid to form a spinning solution, spinning the titania sol and calcining the resultant to obtain a titania fiber (U.S. Pat. No. 4166147); PA1 (4) a method comprising the steps of adding water and hydrochloric acid to an alcohol solution of titanium tetraisopropoxide to hydrolyze the propoxide, carrying out polycondensation to form a spinning solution and spinning the resulting condensation product to obtain a titania fiber (a so-called sol-gel method)(JP-A-62-223323); PA1 (5) a method comprising the steps of reacting a titanium alkoxide with an aliphatic dicarboxylic acid in a solvent to prepare a polymer, concentrating the reaction mixture, spinning the polymer and calcining the resultant to obtain a titania fiber (JP-A-60-104133); PA1 (6) a method comprising the steps of wet-spinning an arginic acid solution to form a continuous fiber, immersing the continuous fiber in a titanium solution, drying the continuous fiber with stretching and calcining to produce a titania fiber (JP-A-2-184525); and PA1 (7) a method comprising the steps of impregnating an organic fiber with an aqueous titanium alkoxy hydrogen peroxide solution and calcining the fiber to it produce a titania fiber (JP-A-2-19569).
It has proven difficult to produce a so-called continuous fiber of titania having a length of at least several tens centimeters, excellent spinning stability and excellent mechanical strength in an industrially easy manner by the above-disclosed conventional methods. Additionally, the conventional methods suffer from various drawbacks.
According to method (1), the resulting fiber is a so-called short fiber wherein a fiber length is normally not more than 1 mm, and at most about several millimeters. It is impossible to produce a continuous fiber. According to method (3), a water-miscible titania sol is used to form a spinning solution. The use of the high-concentrated inorganic acid (e.g. concentrated hydrochloric acid, etc.) to produce the sol restricts the choice of material for the vessel and means that chlorine derived from concentrated hydrochloric acid remains as an impurity contaminating in the resulting fiber.
According to the sol-gel method (4), titanium alkoxide is hydrolyzed and polycondensated in the presence of an acid such as hydrochloric acid to obtain a viscous solution having suitable spinnability, which is used as a spinning solution. In the method (4), it is difficult to control the hydrolysis reaction and the polycondensation reaction and the spinnable viscous solution is easily converted into a solution which can not be spun.
According to method (5) the residual amount of an organic component in the polymer is large and the content of the organic component in the precursor fiber is necessarily high as a result of reacting a titanium alkoxide with an aliphatic dicarboxylic acid in a solvent to prepare a polymer, since an organic group exists in a side chain and between titanium atoms of the polymer. According to the method (6) of wet-spinning an arginic acid solution and immersing the resulting fiber in a titanium solution and according to the method (7) of impregnating an organic fiber with an aqueous titanium alkoxy hydrogen peroxide solution, the residual amount of an organic component in a precursor fiber is large. When a titania fiber is obtained by calcining the precursor fiber containing a large residual amount of the organic component, there is a problem that the mechanical strength of the resulting fiber is low.
Since there is no need to use an organic polymer and a binder according to the method (2), a continuous fiber having high mechanical strength to some degree is obtained. However, a continuous fiber having satisfactory mechanical strength is not always obtained.
As described above, a conventional titania fiber does not satisfy required characteristics criteria for a continuous fiber of titania having excellent spinning stability and high mechanical strength. Additionally, for example, when used as a catalyst carrier, it is particularly required that specific surface area and pore volume of the fiber are high. Nevertheless, those of the conventional titania fiber are not high. When used as a catalyst carrier for reduction of a nitrogen oxide, it is particularly required that the crystal form is an anatase. Nevertheless, it is very difficult to obtain a titania fiber having this crystal form in the conventional methods.
Even in a method comprising steps of immersing a titania fiber in an acid to partially corrode the surface of the fiber for the purpose of increasing the specific surface area, which is based on a conventional method of using a fiber of silica, alumina, etc. as a catalyst carrier (described in JP-A-50-87974 AND JP-B-8-11196), there are problems that it is difficult to form uniform pores on the surface of the fiber and the mechanical strength of the fiber is quite low as compared with a fiber of silica and/or alumina, and that this partially corroding method itself is complicated.
On the other hand, as a method of carrying a catalyst component on and/or in a titania fiber, the following methods are also known but all methods are not sufficient.
For example, JP-A-5-184923 discloses that a vanadium oxide-carrying titania fiber is obtained by heat-treating an amorphous fiber to deposit a crystal of an anatase-form titanium oxide and a crystal of a vanadium oxide. In this method, the amorphous fiber was produced by a sol-gel method of hydrolyzing a alkoxide in a solution of a titanium alkoxide and a vanadium compound or hydrolyzing alkoxides in a solution of a titanium alkoxide, the other alkoxide and a vanadium compound, followed by gelation. However, the titania fiber obtained by this method has problems in that, since an amorphous titanium oxide phase and anatase-form titanium oxide phase coexist, the shape of the fiber can not be sufficiently retained and the catalystic activity removing a nitrogen oxide is low.
JP-A-6-134306 discloses that a catalyst-carrying titania fiber is obtained by forming a polymer including titanium and silicon from organic alkoxides of titanium alkoxide and a silicon alkoxide by a sol-gel method, spinning to form a fiber, drying and calcining the fiber to obtain a fiber of TiO.sub.2 --SiO.sub.2, and carrying vanadium pentaoxide and/or tungsten oxide. According to this method, a titania fiber having an anatase crystal is obtained, however, physical properties such as specific surface area and pore volume are not satisfactory, necessarily.